Heater for a device for providing a liquid additive

ABSTRACT

The invention relates to a heater for a device for providing a liquid additive in a motor vehicle, comprising at least one switchable resistance heating element, which is connected in series with at least one switch element, by means of which the at least one resistance heating element can be activated and deactivated, and comprising at least one permanent resistance heating element, which is connected in parallel with the at least one switchable resistance heating element and the at least one switch element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of PCT Application PCT/EP2015/061687, filed May 27, 2015, which claims priority to German Application DE 10 2014 107 519.6, filed May 28, 2014. The disclosures of the above applications are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a heater for a device for supplying a liquid additive. A device for supplying a liquid additive may be used for example in a motor vehicle in order to feed a liquid additive for exhaust-gas purification to an exhaust-gas treatment device of the motor vehicle.

BACKGROUND OF THE INVENTION

Exhaust-gas treatment devices in which a liquid additive for exhaust-gas purification is used are widely used in particular in motor vehicles with diesel internal combustion engines. The exhaust-gas purification method of selective catalytic reduction (SCR) is commonly implemented in exhaust-gas treatment devices of said type. In said method, nitrogen oxide compounds in the exhaust gas are reduced, with the aid of the liquid additive, to form non-hazardous substances (water, carbon dioxide and nitrogen). For the SCR method, urea-water solution is commonly used as liquid additive. Urea-water solution as liquid additive for exhaust-gas purification is available for example under the trade name AdBlue®, with a urea content of 32.5 wt. %. A urea-water solution of said type is commonly also referred to as reducing agent precursor or reducing agent precursor solution. For the exhaust-gas purification, urea-water solution is converted into ammonia. Said conversion to ammonia may take place outside the exhaust gas in a reactor provided for the purpose, or in the exhaust gas within the exhaust system. The actual reduction reaction of the nitrogen oxide compounds in the exhaust gas takes place with the ammonia.

A problem with regard to the supply of liquid additives for exhaust-gas purification in a motor vehicle is that the commonly used liquid additives can freeze at low temperatures.

The described urea-water solution, for example, freezes at −11° C. In motor vehicles, such low temperatures may arise in particular during long standstill phases of the motor vehicle in winter.

It is known that devices for supplying liquid additive can be heated in order to counteract the freezing problem. By means of the heating, it is firstly possible for the liquid additive to be prevented from freezing. At the same time, by means of the heating, it can be ensured that a device can supply liquid additive quickly during a starting phase in winter, because frozen additive is thawed by the heating.

A problem in the case of the heating of liquid additive is that the liquid additive should not exceed a maximum temperature. For the described urea-water solution with a urea content of 32.5 wt. %, an upper limit temperature is for example approximately 110° C., in particular 80° C. or 60° C. Above said (upper) limit temperature, conversion processes within the liquid additive may commence. Here, the expression “conversion process” encompasses in particular the precipitation of solid particles out of the liquid additive, or the (partial) conversion of liquid additive into ammonia. Precipitated solid particles are for example crystalline urea precipitates, which can form deposits in the device.

What is known, therefore, is the use of PTC (positive temperature coefficient) heating elements for heating devices for supplying liquid additive. In the case of PTC heating elements, the heating power drops very considerably at high temperatures (above a material-dependent limit temperature). The limit temperatures may be defined by means of a suitable configuration of the PTC heating elements. PTC heating elements are normally composed of barium titanate. A problem with the use of PTC heating elements is that they are relatively expensive. Furthermore, in the case of PTC heating elements, an exact limit temperature often cannot be set very precisely. In particular, it is generally not possible to produce a large number of PTC heating elements with exactly consistent limit temperatures.

Furthermore, for the heating of a device for supplying liquid additive, it must be noted that there is commonly a very limited availability of electrical heating energy in a motor vehicle. In motor vehicles, electrical energy is commonly produced in cumbersome fashion by means of an alternator. The heating of a device for supplying liquid additive should therefore require as little electrical energy as possible.

A heater for a device for supplying a liquid additive should furthermore be capable of being activated very quickly in order to heat liquid additive quickly upon a start of a motor vehicle and also, in the case of the liquid additive in the device or in a tank being in a frozen state, to quickly make it possible for liquid additive to be supplied for exhaust-gas purification.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to solve, or at least lessen, the technical problems discussed. For this purpose, it is sought to disclose a particularly advantageous heater for a device for supplying a liquid additive.

These objects are achieved by means of a heater according to the features of claim 1. Further advantageous refinements of the heater are specified in the dependent claims. The features specified individually in the claims may be combined with one another in any desired technologically meaningful way and may be supplemented by explanatory facts from the description and the figures, with further design variants of the invention being specified.

The invention relates to a heater for a device for supplying a liquid additive in a motor vehicle, having at least one switchable resistance heating element which is connected in series with at least one switch element, by means of which the at least one first resistance heating element can be activated and deactivated, and having at least one permanent resistance heating element which is connected in parallel with respect to the at least one switchable resistance heating element and the at least one switch element.

The switchable resistance heating elements and permanent resistance heating elements connected in parallel with respect to one another are preferably connected to a common power source, which can be activated and deactivated. By means of the common power source, it is possible for both the switchable resistance heating elements and also the permanent resistance heating elements to be (jointly) activated and deactivated. The switch elements, which are connected in series (only) with the switchable resistance heating elements, also make it possible for permanent resistance heating elements to be operated when the switchable resistance heating element is deactivated.

In the case of a device for supplying liquid additive which has a heater, it is normally possible to identify a heat transfer coefficient that describes the extent to which heat is transferred from the heater into the liquid additive. The heat transfer coefficient describes the heat flow from the heater into the liquid additive as a function of a temperature difference between the heater and the liquid additive. The heat transfer coefficient is dependent on the thermal conductivity of the individual components of the device.

Furthermore, in a device of said type, it is normally also possible to identify a heat capacity that describes the capability of the device to store heat. The heat capacity describes what fraction of the heat produced by a heater is initially consumed for heating the device before heating of the liquid additive occurs. The heat capacity is dependent on the specific heat capacity of the individual components of the device.

The heat transfer coefficients and the heat capacity result in a time-dependent temperature difference between the heater and the liquid additive in the device during the heating process, which time-dependent temperature difference can be described and determined for example by means of a differential equation or even a system of differential equations.

It has already been described further above that temperatures above a limit temperature (for example a maximum of 110° C. or a maximum of 80° C. or 60° C.) should not arise even locally during the heating of liquid additive, in order that a conversion of the liquid additive is prevented. The at least one permanent resistance heating element is preferably dimensioned such that, even in the case of (permanent) operation of the permanent resistance heating element, no temperatures that exceed such a limit temperature arise and/or can be generated within the device and in the liquid additive. This may be realized for example by virtue of the at least one permanent resistance heating element being correspondingly dimensioned as a function of the heat capacities and the heat transfer coefficients. It is preferable for the possible heating power of the at least one permanent resistance heating element to be so low in relation to the heat capacity and the heat transfer coefficient of the device that, during the operation of the heater, temperatures above the limit temperature cannot arise even locally when the switchable resistance heating element is deactivated.

The at least one switchable resistance heating element makes it possible to provide very high levels of heating power in order to quickly heat a device for the liquid additive.

By means of the described heater, it is possible to dispense with the use of PTC materials.

The heater is particularly advantageous if the electrical resistances of the at least one switchable resistance heating element and of the at least one permanent resistance heating element are configured such that a proportion of between 70% and 95% of the heating power of the heater is generated by the at least one switchable resistance heating element when the at least one switchable resistance heating element is activated.

Accordingly, the at least one permanent resistance heating element generates a proportion of between 5% and 30% of the heating power. It is preferable for the proportion generated by the permanent resistance heating element to be between 8% and 15% of the heating power, and for the proportion generated by the switchable resistance heating element to accordingly be between 85% and 92% of the heating power. It has been found that, by means of such a configuration of the permanent resistance heating element and of the switchable resistance heating element, it may be achieved that instances of local overheating (local exceedances of the limit temperature) can be prevented in an effective manner.

The heater is furthermore advantageous if it has at least one temperature sensor and at least one control component by means of which the at least one switch element and the at least one switchable resistance heating element can be activated and deactivated as a function of a temperature determined by means of the at least one temperature sensor.

Here, the at least one switchable resistance heating element is deactivated by the at least one switch by virtue of an electrically conductive connection being broken by the switch. When activated, the switch produces an electrically conductive connection. The at least one temperature sensor provides temperature information to the control component, and the control component activates and deactivates the switch element as a function of said temperature information.

By means of a temperature sensor, it is for example possible for the temperature of the liquid additive and/or the temperature of the heater to be determined in order to identify (local) overheating of the liquid additive. The control component may for example be an electronic component. The control component may also be a constituent part of a control unit of a motor vehicle. The control component is preferably connected to the temperature sensor and to the at least one switch element via an (electrical) signal line.

Different types of temperature sensors may be used. The temperature sensor may for example be a resistance temperature sensor, the electrical resistance of which varies as a function of the temperature. The temperature sensor may for example also be an infrared sensor within a device for supplying liquid additive, by means of which infrared sensor the temperature can be determined indirectly (by way of infrared radiation).

Temperature sensors are a particularly effective means for identifying overheating of the liquid additive in a device for supplying liquid additive and, by way of regulation of the heating power, reducing or preventing said overheating in an effective manner.

Alternatively and/or in addition to the temperature information from the at least one temperature sensor, the at least one control component may also receive further information that is taken into consideration for the activation and deactivation of the switch element. For example, a timer may be provided which provides time information to the control component. It may for example be provided that when the heater is activated, the switchable resistance heating element is only ever operated for a certain predefined time period of, for example, between 5 and 20 minutes.

The heater is advantageously designed in the manner of a heating foil, wherein the at least one switchable resistance heating element and the at least one permanent resistance heating element are arranged as conductor paths on the heating foil.

A heating foil is normally a sheetlike structure with a (main) surface. A heating foil may for example lie against a tank wall of a tank for the liquid additive and thus heat the liquid additive in a tank over a large area. This permits particularly effective heat transfer from the heater to the liquid additive. Conductor paths for resistance heating elements may be produced on the heating foil for example by way of printing processes. Such conductor paths may also be etched into a conductive material on the heating foil.

It is particularly preferable for at least two electrical contacts to be arranged on the heating foil, via which electrical contacts power is supplied to the heater. Electrical contacts on the heating foil may likewise be printed. It is also possible for metallic contacts to be adhesively bonded or soldered on. In one advantageous design variant, the at least two electrical contacts are formed by solder spots at which electrical supply lines can be soldered to the heating foil.

In a further particularly advantageous design variant of the heating foil, at least one of the following components is likewise arranged on the heating foil:

at least one switch element,

at least one (electronic) control component,

at least one temperature sensor.

Such switch elements and electronic control components or temperature sensors may optionally likewise be arranged on the heating foil using the same production methods (for example printing processes and/or etching), for example with the aid of printing processes.

It is also particularly advantageous for the heating foil to be flexible. A flexible heating foil can for example be adapted to differently (in particular irregularly) shaped regions of a tank or of a tank wall. A flexible heating foil may for example have a flexible (for example rubber-like) carrier material on which the individual components (resistance heating elements, switch elements, control components etc.) are arranged.

The heater furthermore advantageously has multiple switchable resistance heating elements and multiple switch elements assigned to in each case one switchable resistance heating element, wherein the switchable resistance heating elements are arranged in each case in different surface sections of the heater.

A heater of said type preferably has an areal extent with a surface, and is particularly preferably a heating foil. By means of multiple surface sections which each have a switchable resistance heating element, it is possible for the heating power of the heater to be locally (individually) adjusted. It is accordingly possible for higher levels of heating power to be provided in regions of the heater that are at low temperature than in those regions of the heater at which higher temperatures prevail. An individual adaptation of the heating power to the respectively (locally) present conditions is thus possible.

A heater of said type is particularly advantageous if at least one switchable resistance heating element is assigned at least one temperature sensor, and said switchable resistance heating element can be activated and deactivated, as a function of a temperature determined by means of the at least one temperature sensor, with the aid of at least one assigned switch element and at least one control component.

It is preferable for an arrangement of precisely one temperature sensor to be provided for each switchable resistance heating element, by means of which temperature sensor temperature information can be obtained for the switchable resistance heating element in order for a switch element of the switchable resistance heating element to be activated and deactivated as a function of the temperature information.

It is also possible for precisely one control component to be provided for each switchable resistance heating element. It is however alternatively also possible for the temperature information from multiple temperature sensors to be processed (centrally) in one control component which is set up to control (activate and deactivate) multiple switch elements. It is preferable for the temperature sensors to each be arranged centrally in the region of the respective switchable resistance heating element. By means of an arrangement of said type, it is possible in a particularly effective manner for the temperature at the respective switchable resistance heating elements to be identified and for said switchable resistance heating elements to then be activated and deactivated in a targeted and individual manner.

The at least one switchable resistance heating element, the at least one switch element and the at least one control component of the heater may also be set up for clocked operation, wherein the at least one switchable resistance heating element is in each case activated briefly and deactivated again in order to generate reduced levels of heating power. It is particularly preferably possible for the at least one switchable resistance heating element to be operated by way of pulse width modulation (PWM). By means of such operation, the heating power of the at least one switchable resistance heating element can be reduced in continuously variable fashion.

The invention proposes a device for supplying a liquid additive in a motor vehicle, having a tank for storing the liquid additive, said tank having a tank wall, wherein at least one described electric heater is arranged on an outer side of the tank wall such that liquid additive in the tank is heated through the tank wall.

The special advantages and design features described in conjunction with the described heater can be transferred analogously to the device. The same applies to the special advantages and design features described below for the device, which can be transferred analogously to the heater.

The device preferably comprises a delivery module which has a housing which is integrated into the tank wall of the tank in a base of the tank. For this purpose, the base of the tank wall preferably has an opening into which the housing is inserted, such that the housing closes off the opening. The housing then extends into an interior space of the tank. The housing then forms a section of the tank wall. An inner side of the housing then forms an outer side of the tank wall. The described heater is particularly preferably arranged on the inner side of the housing and is thus capable of heating the liquid additive in the tank through the tank wall formed by the housing.

It is particularly advantageous in this context for the heater to be a flexible heating foil which can be adapted to the shape of the housing.

Also described here is a motor vehicle having an internal combustion engine and having an exhaust-gas treatment device for the purification of the exhaust gases of the internal combustion engine, and a device for supplying liquid additive for the exhaust-gas treatment device, said device having a described electric heater.

The invention and the technical field will be explained in more detail below on the basis of the figures. The figures show particularly preferred exemplary embodiments of the invention, to which the invention is however not restricted. In particular, note that the proportions illustrated in the figures are merely schematic.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1: shows a first design variant of a described heating foil,

FIG. 2: shows a second design variant of a described heating foil,

FIG. 3: shows a device having a described heating foil,

FIG. 4: shows a section through a housing of a delivery unit having a described heating foil, and

FIG. 5: shows a motor vehicle having a device for supplying liquid additive, said device having a described heating foil.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

FIGS. 1 and 2 each illustrate the heater 1, which is in the form of a heating foil 9. It can be seen that the heating foil 9 has a surface 10.

FIG. 1 and FIG. 2 each show electrical contacts 12 by way of which the heater 1 can be supplied with power. Also shown are conductor paths 11 on the heating foil, which conductor paths form switchable resistance heating elements 4 and permanent resistance heating elements 6. Also shown are switch elements 5, (electronic) control components 8 and temperature sensors 7.

In the design variant of FIG. 1, precisely one permanent resistance heating element 6 and one switchable resistance heating element 4 are provided.

In the design variant of FIG. 2, in each case two supply conductor paths 23 are provided via which multiple switchable resistance heating elements 4, which each have a switch element 5, an electronic control component 8 and a temperature sensor 7, can be energized. Furthermore, in the design variant in FIG. 2, there is precisely one permanent resistance heating element 6. It can be seen that the individual switchable resistance heating elements 4 are arranged in each case in surface sections 13 of a surface 10 of the heater 1.

The design variants of the heater 1 illustrated in FIGS. 1 and 2 are in particular not restricted with regard to the arrangement of temperature sensors 7, control components 8 and switch elements 5. Said elements (switch elements 5, control components 8 and temperature sensors 7) may also be arranged outside the heater 1, for example on other components of a device for supplying liquid additive.

Furthermore, FIGS. 1 and 2 are not restricted with regard to the arrangements and number of switchable resistance heating elements 4 and permanent resistance heating elements 6 illustrated therein. Resistance heating elements may be arranged on the heating foil as desired. What are particularly suitable in each case are resistance heating elements that run in meandering fashion, because by means of meandering structures, it is possible to realize relatively high electrical resistances and a uniform distribution of the electrical heating power.

FIG. 3 shows a tank 15 for a liquid additive, which tank has a tank wall 29. A housing 14, in which a pump 22 is situated, is integrated into the tank 15 in the region of a tank base. The pump 22 can extract liquid additive 28 from the tank 15 at an intake point 20. Said liquid additive is supplied by the pump 22 to a supply port 21. On the inner side 16 of the housing 14 there is situated a described heater 1, which extends in areal fashion along the inner side 16 of the housing. In

FIG. 3, the liquid additive is present in the tank in the form of frozen additive 27. Within the frozen additive 27 there is formed an ice cavity 19 which has been generated by the heater 1 by virtue of the frozen additive 27 having been melted to form liquid additive 28.

FIG. 4 shows the section A-A, indicated in FIG. 3, through the housing 14 of the device 2. It is possible to see the heater 1 with the electrical contacts 12 and the pump 22 and the intake point 20. The heater 1 lies against an inner side 16 of the housing 14, wherein the inner side of the housing 14 forms an outer side 30 of the tank 15.

FIG. 5 shows a motor vehicle 3 having an internal combustion engine 17 and having an exhaust-gas treatment device 18 for the purification of the exhaust gases of the internal combustion engine 17. In the exhaust-gas treatment device 18 there is arranged an SCR catalytic converter 26 for carrying out the method of selective catalytic reduction. A liquid additive can be fed from a tank 15 to the exhaust-gas treatment device 18 by means of a metering device 25. Liquid additive is supplied to the metering device 25 via a line 24 by a device 2 arranged in the tank 15.

The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention. 

1. A heater for a device for supplying a liquid additive in a motor vehicle comprising: at least one resistance heating element; at least one switch element; at least one switchable resistance heating element which is connected in series with the at least one switch element, where the at least one switch element is used to activate and deactivate the at least one switchable resistance heating element; and at least one permanent resistance heating element which is connected in parallel with respect to the at least one switchable resistance heating element and the at least one switch element.
 2. The heater according to claim 1, wherein the electrical resistances of the at least one switchable resistance heating element and of the at least one permanent resistance heating element are configured such that a proportion of between 70% and 95% of the heating power of the heater is generated by the at least one switchable resistance heating element when the at least one switchable resistance heating element is activated.
 2. The heater according to claim 1, further comprising: at least one temperature sensor; and at least one control component used to control the at least one switch element and the at least one switchable resistance heating element as a function of a temperature determined by the at least one temperature sensor.
 3. The heater according to claim 1, further comprising: a heating foil with a surface; wherein the at least one switchable resistance heating element and the at least one permanent resistance heating element are arranged as conductor paths on the heating foil.
 5. The heater according to claim 3, wherein the heating foil (9) is flexible.
 6. The heater according to claim 1, further comprising a plurality of switchable resistance heating elements and a plurality of switch elements assigned to a corresponding one of the plurality of in switchable resistance heating elements, wherein the plurality of switchable resistance heating elements are arranged in different surface sections of the heater.
 7. The heater according to claim 6, wherein each of the plurality of switchable resistance heating elements is assigned at least one corresponding temperature sensor, and each of the plurality of switchable resistance heating elements are activated and deactivated, as a function of a temperature determined by the at least one corresponding temperature sensor, and one of the plurality of switch elements and at least one control component.
 8. The heater according to claim 1, further comprising a device for providing liquid additive in a motor vehicle, wherein the at least one electric heater is used to heat the liquid additive in the device.
 9. (canceled)
 10. The heater according to claim 8, the device for providing liquid additive to a motor vehicle further comprising a tank for storing the liquid additive; and a tank wall, the tank wall being part of the tank; wherein the at least one electric heater is arranged on an outer side of the tank wall such that liquid additive in the tank is heated by the at least one heater through the tank wall.
 11. The heater according to claim 1, further comprising an exhaust gas treatment device, the exhaust gas treatment device being part of the motor vehicle.
 12. The heater according to claim 11, the motor vehicle further comprising an internal combustion engine, the exhaust-gas treatment device for purifying the exhaust gases of the internal combustion engine; and a device for supplying liquid additive for the exhaust-gas treatment device, the heater being used with said device. 